JOLIET, Ill. (AP) — Mark Ruffalo had a big smile on his face as he signed ticket stubs and posed for pictures at Chicagoland Speedway. A steady stream of fans approached the actor from all angles, but he made it look as if it was a small party with a bunch of old friends.

Ruffalo felt right at home at his second NASCAR race.

"This culture is really familiar to me," he said. "These are like my people. I grew up an hour and a half away from here in Kenosha, Wisconsin. So this is my kind of family. This is what my people are like."

Ruffalo, 46, served as the grand marshal for Saturday night's Nationwide Series stop, but he also came out to support Leilani Munter in the ARCA race. Munter's PrairieGold Solar No. 66 Toyota included an ad for Ruffalo's The Solutions Project on the car.

Ruffalo and Stanford professor Mark Jacobson helped create the organization in 2011 to advocate for the transition to 100 percent clean, renewable energy. NASCAR has stepped up its environmental efforts over the years, and Ruffalo said he found a receptive audience at the race

It helped that Ruffalo had the perfect spokeswoman in Munter, who has a degree in biology and drives an electric Tesla Model S off the track. Munter also has solar panels on her house, and her motto is "Never underestimate a vegetarian hippie chick with a race car."

Using a solar-based system in her pit area that is also used by the U.S. military, Munter finished 12th in the ARCA race.

"What we want to say to people and what she is a perfect example of is you know we really don't have to give up anything," Ruffalo said. "You don't have to give up NASCAR because we're moving to a carbon-free structure and infrastructure of carbon-free energy environment. You can still have those things."

Ruffalo's newest movie is "Begin Again," in which he plays a divorced, middle-aged record executive struggling in the modern music industry. It also stars Keira Knightley as a British singer who captures Ruffalo's attention, and Hailee Steinfeld as his teenage daughter.

He received an Emmy nomination this month for his compelling performance in "The Normal Heart," an HBO film adapted from Larry Kramer's 1985 play about the early years of the AIDS crisis.

"It was probably one of the most important things I've done," Ruffalo said. "It had such a responsibility to get that story right for the millions of people who have died since that time, a lot of them senselessly, and then the survivors."

One of his current projects is "Avengers: Age of Ultron," which is scheduled for release on May 1, 2015. Ruffalo, who plays The Hulk, said the sequel to the Joss Whedon-directed 2012 blockbuster finds the group of superheroes in a tough spot following the end of Shield, the organization that brought them together in the first place.

"Joss is in his full height of his powers. This movie is much more sprawling and epic than the first one," he said. "They're really in an existential, dire crisis. It has a darker quality. The Avengers are really struggling with their own shortcomings and the dissolution of Shield and where they go and who they are in the world. There's a lot of negativity towards them."